Mike McCartney, Cousins' agent, announced his client signed a three-year deal with the Vikings.
Per NFL Network's Ian Rapoport, Cousins received a fully-guaranteed contract worth $84 million and his average annual salary of $28 million is the highest in NFL history.
Per The MMQB's Andrew Brandt, McCartney also noted Cousins' contract breaks down to $26 million in 2018, $28 million in 2019 and $30 million in 2020.
ESPN's Adam Schefter first reported on Tuesday that Cousins intended to sign with the Vikings. Before putting pen to paper, he traveled to Minnesota for a tour of the Vikings' headquarters and training facility.
Cousins' $84 million in guaranteed money is the third-most in the NFL, behind Detroit Lions quarterback Matthew Stafford ($92 million) and Indianapolis Colts quarterback Andrew Luck ($87 million), per Spotrac.
The Vikings reached the NFC Championship Game last season with Case Keenum as their starting quarterback.
Cousins has thrown for at least 4,000 yards and 25 touchdowns in each of the previous three seasons with Washington. The 29-year-old led the NFL with a 69.8 completion percentage in 2015, his first year as a full-time starter.